1. Field of the Invention
This invention relates to a method and apparatus for performing terahertz (THz) spectroscopy and, more particularly, for optically pumping and THz probing a sample object under test by a non-linear technique.
2. Description of the Related Art
Over the years, the trend towards smaller scales in Metal-Oxide Semiconductor Field-Effect Transistors (MOSFETs) and other CMOS based technologies has brought about substantial improvements in devices performance. However, the silicon-based microelectronics industry is rapidly approaching the point where device performance is compromised by feature size. Specifically, the gate dielectric based on SiO2 scaled to a node size of 90 nm will not insulate well enough, thereby leading to gate leakage. Current MOSFET technology with 1.3 nm thick SiO2 as the gate dielectric material portrays high gate leakage and reduced drive currents, thus limiting the operation range of the device and also increasing power consumption. Gate leakage currents however, can be minimized by using other dielectric materials with higher permittivity values than their SiO2, counterpart. These other dielectric materials exhibit smaller electric fields under the same bias conditions than does SiO2.
SiO2 has been the industry workhorse for many years. It exhibits extremely low defects when grown on Si substrate and therefore has little effect on the performance of the device. Despite many efforts, a high-κ dielectric material that provides as suitable an interface with silicon as with SiO2 has not been found. However, manufacturing processes that incorporate dielectric composites such as hafnium-silicate (HfSiO) or plasma-nitride hafnium-silicate (HfSiON) have provided improved oxide interfaces for device manufacturing. Among the factors that contribute to a poor interface between the oxide and the Si substrate are thermodynamic stability with silicon at elevated temperatures, the dielectric constant, and the silicon conduction band offset. Herein, high-κ dielectric material refers to material having a dielectric constant higher than that of SiO2. More specifically, high-κ dielectric material may have a dielectric constant higher than 3.9.
A large number of gate dielectric materials have been examined during the past few years. Films such as HfO2 HfSixOy have gained much attention for their potential applications in advanced microelectronics. Various methods have been developed to deposit these high-κ dielectric materials, such as chemical vapor deposition, physical vapor deposition, and ion-assisted deposition. Hf-based high-κ dielectrics have become a very promising candidate to replace SiO2 in MOSFETs due to their adequate band-gaps and conduction-band offsets. (See, Lin, Y.-S., Puthenkovilakam, R., and Chang, J. P., “Dielectric property and thermal stability of HfO2 on silicon,” Appl. Phys. Lett. 81, 2041-2043 (2002) and Robertson J 2000 J. Vac. Sci. Technol. B 18 1785). A dielectric constant around 25, thermodynamic stability with Si up to 950° C., and a conduction band offset of 1.5 eV allow it (Hf-based dielectric material) to offer a sufficient barrier against electron tunneling with an equivalent oxide thickness (EOT) as small as 10 angstroms. If the capacitor dielectric is SiO2, a capacitance density of 34.5 fF/μm2 would correspond to an equivalent oxide thickness teq of 10 Å. Equivalent oxide thickness represents the theoretical thickness of SiO2 that would be required to achieve the same capacitance density as the dielectric ignoring issues such as leakage current and reliability. HfO2 with a bulk relative permittivity of 25, therefore affords a physical thickness of 63 Åto obtain teq of 10 Å. Deducing the effect of an oxide on the electronic mobility and other parameters within the silicon underneath it using all-optical methods involving either linear and/or differential spectroscopic measurements would be beneficial because it would promote in-situ and non-contact measurement of the pertinent characteristics and because it would facilitate online quality monitoring of fabricated microelectronic components.
Terahertz spectroscopy has been used to assess electronic properties of materials. Optical pump—THz probe time-resolved techniques has been used to study the dynamics of mobile charge carriers in materials before they are trapped at defect sites. In such techniques, it is well understood that both the optical pump and the THz probe are pulsed. The optical pump pulse causes photo-excitation of the carriers, while the THz probe pulse measures the carrier dynamics.
These techniques have been reportedly used on a wide variety of sample materials from semiconductors to superconductors and the like to study carrier dynamics. But none of these techniques has been able to provide an accurate estimate of defects in materials as set forth below.